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Oil and Water DO Mix - The Science of Mayonnaise

Pull out your jar of Hellmanns’ or whatever, and read the ingredient statement. Remember that ingredients are listed in order by amount in the product. The first one on my jar is “soybean oil”, followed by “water”. So what gives? Why do not the oil and water separate like they do when you make oil and vinegar (vinegar is about 95% water) dressing? The answer is the third ingredient, the egg. Specifically, egg yolk.

Egg yolk contains, amongst other things, a material called “lecithin”, which is an emulsifying agent, like soap. Lecithin molecules have one end that is water soluble (”hydrophilic” in chemistry lingo) and another end that is oil soluble (”hydrophobic). When egg yolk is mixed with oil and water and stirred or beaten vigorously, the lecithin literally sticks its hydrophilic end in the water droplets, making a sphere of lecithin spikes sticking out of the small water droplets. The ends sticking out are hydrophobic, and dissolve in the oil. (It would work the other way, but due to how mayonnaise is made, it the water that gets the lecithin first). These aggregates of water and lecithin are called “micelles” and essentially hide the water in an oil soluble material. Read more »